Exposure apparatus, laser measurement apparatus or the like are accommodated in a chamber formed for example in a clean room, and the position of an X-Y stage thereof is accurately measured with a laser interferometer.
With this measurement using a laser interferometer, since the laser wavelength changes due to changes in temperature or humidity, the temperature and humidity inside the chamber is controlled by an air conditioner.
However, if air disturbances occur along the interferometer beam optical path, the refractive index of the air in the optical path changes, or the laser wavelength changes so that consequently there is a drop in measurement accuracy.
For example, in the case where the temperature, pressure and humidity along the optical path respectively change as .DELTA.T (.degree. C.), .DELTA.P (hPa), .DELTA.H (%), then the relative change amount .DELTA..lambda./.lambda. of the laser wavelength .lambda. can be approximately expressed by the following equation: EQU .DELTA..lambda./.lambda.=(0.93.DELTA.T+0.27.DELTA.P-0. 0098.DELTA.H).times.10.sup.-6
At this time, the measurement error for a measurement length L is according to the following equation: EQU .DELTA.L=L.times.(.DELTA..lambda./.lambda.)
For example for a measurement length L=50 mm, in order to permit a measurement error .DELTA.L &lt;1 nm (10.sup.-9 m), the following must apply:
.DELTA.T&lt;0.022.degree. C. PA1 .DELTA.P&lt;0.074 hPa PA1 .DELTA.H&lt;2.01%
(here only one parameter has been calculated, with other parameters fixed for convenience) With present state chambers the following specifications are met:
Temperature accuracy ripple value=0.02.degree. C. or less
Humidity accuracy ripple value=0.4.degree. C. or less Hence for the temperature and humidity, these are within tolerance.
However, control of pressure inside the chamber is not at present performed.
Atmospheric pressure is the weight of air per unit area. In the normal range of 800.about.1100 hPa, this continuously changes with the meteorological situation. Normally, with the chamber, the sealing is high, however if there is a gap of pin hole order, the chamber interior and the outside air side are effectively connected pressure wise. Therefore the inside-chamber pressure immediately fluctuates with fluctuations in the external atmospheric pressure. In particular, it is known that for example on windy days, fast fluctuations with periods of several seconds are seen (short period fluctuations), so that the maximum amplitude inside the chamber is as much as 0.4 hPa.
Normally, with long period fluctuations in external atmospheric pressure, these can be corrected by the measuring system software. However, with short period fluctuations in external atmospheric pressure, there is a problem that these cannot be fully corrected.
Furthermore, it has also been considered to control the inside-chamber pressure to an absolute pressure (constant pressure value) irrespective of fluctuations in external atmospheric pressure. In this case however, if the set absolute inside-chamber pressure is 1000 hPa (1 atm.), then for the chamber, a pressure resistant container where the volume does not change even for fluctuations of .+-.100 hPa (1020 kg/m.sup.2) is required. At present such a chamber does not exist. That is to say, the plate thickness of the chamber wall must be a plate thickness such as for a submarine.
It is therefore an object of the present invention to address the above problems by providing a method and apparatus for controlling the pressure in a chamber which, when the pressure inside a chamber in which laser measurement is being conducted follows long period fluctuations in external atmospheric pressure, can suppress any following of short period fluctuations in external atmospheric pressure contained in the long period fluctuations in external atmospheric pressure.